Diolefin are widely present in FCC gasoline, pyrolysis gasoline and olefin-rich feed-stock. Diolefin are highly reactive and easy to form polymer, gum and coke precursors itself or with other olefins. Therefore, in order to avoid diolefin coking on catalysts and affecting lifetime, it is necessary to remove diolefin before treating gasoline and olefin-rich feedstock.
At present, selective hydrogenation is the main route to remove diolefin in refineries. Namely, diolefin are removed by selective hydrogenation over hydrogenation catalysts to achieve the purpose of removal of diolefin. There are three kinds of selective hydrogenation catalysts for removing diolefin. The first are catalysts loading noble metal (mainly Pd), such as catalysts disclosed in patents U.S. Pat. No. 6,388,612, U.S. Pat. No. 6,255,548, U.S. Pat. No. 6,084,140, CN 101628843 and CN 1071443A, which provided a method to remove diolefin in olefin-rich feed-stock using Pd/α-Al2O3. The patents also pointed that some other metals such as Ag, Cu and Co could also be added to improve the selectivity of diolefin. The Pd content was in the range of 0.05-0.2 wt %, preferably Pd content was 0.2 wt %, and the preferable reaction temperature was 26-49° C. This type of catalysts exhibits high efficiency on feed-stock with low or no toxicants (e.g., S and As). However, the catalysts are easy to be inactive when treating FCC gasoline, pyrolysis gasoline and olefin-rich feed-stock with high toxicants as above mentioned, and so forth, and then the life of the catalysts are seriously affected. Besides, the price of Pd is also expensive.
The second are Ni-based supported or amorphous state Ni-based catalysts, such as catalysts disclosed in patents CN1221638C, CN99120660.6, and CN100566827C. These patents disclosed a method to remove diolefin in olefin-rich feed-stock using Ni-based supported or amorphous state Ni-based catalysts, where the non-oxidizing porous supports are composed of molecular sieves, active carbon, inorganic oxides and so on. The process of removing diolefin has following features: reaction temperature of 40-70° C., pressure of 1.0-3.0 MPa, H2/oil ratio of 100-700 Ncm3/cm3, and LHSV of 0.5-4.0 h−1. The catalysts have high efficiency on feed-stock with low or no toxicants (e.g. S and As). However, the Ni-based catalysts are easy to be poisoned and inactive when treating FCC gasoline, pyrolysis gasoline and olefin-rich feed-stock with high toxicants (e.g. S and As) as above mentioned and so forth, and then the life of the catalysts are seriously affected.
The third are transition metal sulfides supported catalysts. Chinese patents CN1676580A, CN101619236A, CN100338190C, CN1317366C, CN1317365C, CN1286951C, CN1291785C and CN1272103C had published a method to selectively remove diolefin in distillate using catalysts which were composed of Ni(Co)—Mo(W)—K/Al2O3 and alkali metal (e.g. K). Prior to reaction, it is necessary to sulfide catalysts to form sulfided active species. The operating conditions of diolefin selective hydrogenation have following features: reaction temperature of 160-300° C., preferably 200-260° C.; 1.0-6.0 MPa of H2 pressure, preferably 1.2-4.0 MPa; LHSV of 2.0-30.0 h−1, preferably 5.0-20.0 h−1; and H2/oil ratio of 50-600 Ncm3/cm3, preferably 100-400 Ncm3/cm3. Compared with the two mentioned kinds of catalysts, the third catalysts exhibit high selectivity and good S, As anti-poisoning ability, and existence of alkalis could inhibit carbon deposition on catalyst surface. However, this type of catalyst also has insuperable defects of its own. Due to limited active metals loaded, the catalysts still require high temperature and show low reactivity. Meanwhile, high temperature also accelerates coking, catalysts become inactive easily, and the lifetime of catalysts will decrease greatly, and greatly affects the long-term stable operation of the device. Therefore, it is necessary to develop inexpensive catalysts with high activity and selectivity to remove diolefin under relatively low temperature, while possessing strong S, As anti-poisoning ability and high metal contents, but economic compared with noble metal.
U.S. Pat. Nos. 6,299,760, 6,156,695, 6,783,663, 6,712,955, and 6,758,963 have disclosed a new tri-metallic NiMoW catalyst with high metal content, its preparation and application in ultra-deep hydrodesulfurization of diesel. The HDS activity of the NiMoW catalyst is at least about 3 times of other conventional supported catalysts. The NiMoW catalyst was prepared using ammonia as a cheating agent to react with Ni2+. Via slow heating, the Ni2+ complex in solution of Mo and W would decompose and then the NiMoW precursor was obtained. Sulfided NiMoW catalyst was formed after calcination and sulfidation. The disadvantage of this method is that concentrated ammonia is a pollutant, and the complex of Ni2+ with ammonia was too stable to release ammonia, leaving complex ions of Ni2+ with ammonia in the liquid remnants, and leading to large quantities of waste water that cannot be discharged. The prepared catalysts in these patents possess low surface area (<120 m2/g) and volume (<0.2 ml/g), while in HDS reaction of diesel these catalysts shown high HDS activity only under conditions of high pressure (>6 MPa) and H2/oil ratio (>500 Ncm3/cm3). But when treating olefin-rich feed-stock, the catalysts would lose activity quickly, and this limited the industrial application.
G. Alonso-Nunez et al. in their work (Applied Catalysis A: General 304 (2006)124-130); Applied Catalysis A: General 302 (2006)177-184) and Catalysis Letters 99(2005)65-71)) reported several preparation methods of NiMoW catalysts via different raw materials and various curing agents. The catalysts they prepared had special flaked shape, but the synthesis method was so complex that the steps were also complex and the raw materials were expensive, leading to high costs of catalysts. Moreover, it is also difficult to have a extrusion molding for the sulfided catalyst powder, which limited the industrial application.
Chinese Patent Application Publication No. CN 1339985A also developed a route to synthesize NiMoW catalyst, in which via reaction of Mo, W salts and basic nickel carbonate in water the solid precursor was obtained, and then sulfided the solid precursor. During the procedure at least part of the metal components exists in solid form. Due to using solid Ni source, which is insoluble in water and the essence of synthesis reactions is an ion-exchanged reaction, it is not easy to prepare a catalyst with a small size. The activity of catalyst made no difference from the conventional alumina supported catalysts. CN 101153228A, CN 101544904A and CN 101733120A disclosed a NiMoW trimetallic bulk catalyst, its preparation and use in ultra-deep hydrodesulfurization of diesel, too. Though the bulk catalyst exhibited ultra-high HDS activity in diesel ultra-deep hydrodesulfurization reaction of diesel, it could not be used in removing diolefin from olefin-rich feed-stock, because of low surface area and volume (low carbon capacity). The diolefin declined coking on the catalyst, which would result in a short lifetime of the catalyst, and the diolefin removal could not meet the demand for industrial application.
Based on the existing reports, there are several drawbacks for diolefin removal catalysts as follows: (1) Pd-based and Ni-based supported catalysts with poor S, As anti-poisoning ability and short lifetime, could not treat feedstock containing S and As effectively; (2) the price of Pd-based catalysts is expensive; (3) conventional transition metal sulfided catalysts show low activity, require high temperature and become inactive easily. Therefore, it is necessary to develop inexpensive catalysts to remove diolefin with high activity and selectivity under relatively low temperature, while still possess strong S, As anti-poisoning ability and high metal contents, and economic compared with noble metal.